Transformer



Sept. 13, 1966 M s 3,273,099

TRANSFORMER Filed June 15, 1964 z /9 W; 1 ,5 IO

I I '1 INTERNAL 1,: ,i i [1H COMBUSTION W g" inl% If 1 ENGINE I I 1 H 1/3 ELECTRICAL g [I] 1 6 PULSE MZS/ 22! 1 SOURCE 2 INVENTOR- FLOYD M.MINKS Z7 3/ BY 33 qndrus Star/ e United States Patent 3,273,099TRANSFORMER Floyd M. Minks, Chester, N.H., assignor to KiekhaeferCorporation, Fond du Lac, Wis, a corporation of Delaware Filed June 15,1964, Ser. No. 374,925 14 Claims. (Cl. 33683) This invention relates toa transformer construction and particularly to a high voltage pulsetransformer which may be employed in internal-combustion engine ignitionsystems and the like.

Transformers are often employed to step up a low voltage to a relativelyhigh voltage. The present invention is particularly directed to atransformer which is adapted to take an incoming low voltage pulse andprovide a substantially higher voltage pulse of relatively shortduration. For example, in ignition systems for automobiles and otherinternal-combustion engines, a voltage pulse may be fed through aprimary coil having a secondary coupled thereto to provide a highvoltage pulse to the spark plugs or other firing systems. If the voltageof the output pulse can be increased, the problems of misfiring can besubstantially reduced. Of great practical significance is the rise timeof the pulse; i.e. the time required for the ignition pulse to increasefrom a low value to the value at which a spark occurs across a sparkplug. If this time is short, less energy is lost in shunt resistance.Thus, considering the shunt resistance that eventually develops in apractical system, with two pulses of equal energy, the one with thefaster rise time will have a substantially greater ability to produce aspark and thus ignite the mixed fuel-air mixture.

The present invention is directed to a transformer having thecharacteristics of a high frequency system to provide a fast rise pulseand therefore provide a transformer which is particularly adapted to beemployed in connection with an internal-combustion ignition system orother pulse system where a fast rise time and high voltage is important.

In accordance with the present invention, a core of a substantiallycontinuous magnetic path is provided defining a winding opening with alow voltage primary winding and a high voltage secondary winding woundin stacked relation on a portion thereof. A secondary insulating andwinding bobbin for the secondary encircles a portion of the path withthe bobbin engaging the adjacent core portion toprovide maximum couplingof the secondary winding carried thereby. In assembly, the outer face ofthe bobbin may be provided with a winding groove within which the highvoltage winding is wound, a multiple part bobbin may be assembled with aprewound secondary or the bobbin maybe molded or cast around thesecondary winding. In the latter two constructions, the winding might bewound on a winding tube of insulation, a semi-conductor or a conductorhaving a narrow slit to prevent formation of a shorted turn. The end ofthe tube maybe rounded to reduce the adjacent dielectric stress.

The high .vo-ltagewinding in accordance with the present invention isgenerally a conductive tape member having a width substantiallycorresponding to the width of the winding groove and also having aninsulating coating on at least one surface thereof. The tape is woundwithin the groove upon itself to form a multipleturn spiral windingsubstantially filling the groove. The outer end of the winding isgrounded and the high voltage connection is made to an inner turn of thewinding. The positioning of the high voltage connection at the innerconvolution reduces the capacitance to ground because the innerconvolution is of a minimum area and at the highest voltage. Further, inaccordance with the present invention, a pancake type primary winding iswound about the core immediately adjacent the ends or sides of thebobbin. The primary coil is preferably a multipleturn conductor havingall of the turns in a single plane to provide a planar coil. If desired,a similar primary coil may be mounted on opposite sides of the windingbobbin to minimize leakage inductance. The structure of this inventionprovides, except along the relative long path adjacent the side of thesecondary winding, a continuous insulating mass between the high voltageend of the secondary and all grounded surfaces. Also, the winding of oneturn per layer reduces the thickness of interlayer insulation in thetotal secondary by a factor of approximately two and thus contributes toa small transformer with low leakage inductance and distributedcapacitance. Consequently, very high voltages and good pulse fidelitycan be generated in the secondary winding in a relatively small andcompact transformer.

The drawing furnished herewith illustrates preferred constructions ofthe present invention clearly disclosing the above features andadvantages of the present invention as well as others which will beclear to those skilled in the art.

In the drawing:

FIG. 1 is a side elevational view of a transformer constructed inaccordance with the present invention and connected in an ignitionsystem;

FIG. 2 is a plan view of the transformer shown in FIG. 1;

FIG. 3 is an enlarge-d view with the portions in vertical section;

FIG. 4 is a fragmentary partial view of the winding tape employed in thetransformer of FIGS. 1-3 and typically shows a preferred constructiontherefor;

FIG. 5 is a vertical section similar to FIG. 4 of an alternativetransformer construction in accordance with the present invention; and

FIG. 6 is a bottom view of FIG. 5.

Referring to the drawings and particularly to FIG. 1, an ignition systemis diagrammatically shown for an internal-combustion engine or the likehaving a spark plug 1 forming a part of the engine, which is otherwiseshown in block diagram 2. The spark plug 1 is mounted within an engineblock and in particular includes a spark gap 3 which is disposed withina firing cylinder, not shown. A pulse transformer 4 constructed inaccordance with the present invention has its output connected acrossthe spark gap 3 which will be through a distributor, not shown, formultiple cylinder engines and has its input connected to a suitablepulse source 5 for firing the engine. The pulse source 5 may be of anysuitable or desired construction and is preferably a suitable circuitwhich provides a rapidly rising pulse to the pulse transformer 4 whichin turn provides a rapid rising pulse to the spark plug 3 for ignitingof an explosive mixture.

- Although the pulse transformer 4 is shown in an ignition systembecause of its highly practical adaptation to such application, it maybe used in other circuits which requirea reliable pulse transformerhaving the characteristics described herein.

Generally, the illustrated pulse transformer 4 includes a magnetic coreassembly 6 defining a winding window 6a within which a high voltagesecondary winding 7 is wound. In the illustrated embodiment of theinvention, secondary winding 7 is wound within an insulating and windingbobbin 8 which has a central opening encircling a coupling portion ofthe magnetic core assembly 6. A pancake type primary winding 9 encirclesthe corresponding portion of the magnetic core assembly 6 immediatelyadjacent to one side of the bobbin 8 and is connected to the pulsesource 5. The structure provides a close coupling of the primary winding9 to the secondary winding 7 with a very efiicient transfer of theenergy into the secondary winding 7.

More particularly, the magnetic core assembly 6 includes a pair ofU-shaped similar cores and 11 having side portions of circular crosssections and formed of a high frequency ferrite or other suitable highfrequency core material such as is well known in the transformer art andthe like. The cores 1t) and 11 are mounted with the ends of the sidelegs in alignment and adjacent one another with a slight spacingtherebetween provided by suitable non-magnetic spacers 12. Suitableclamping means such as nut and bolt units 13 extend through edge slotsor grooves in the sides of cores 10 and 11 and are provided forinterconnecting cores 10 and 11 and the non-magnetic spacers 12 providea continuous magnetic path interrupted only by the air gaps defined byspacers 12. The spacers 12 may be eliminated and the cores K10 and 11clamped in touching engagement. A bias or reset winding may thenadvantageously encircle the core assembly and be connected to a suitablecurrent source. The connected cores 10 and 11 define a winding Windowthrough which the secondary winding 7 and the primary winding 9 arewound; in the illustrated embodiment about the left side portion of thegenerally rectangular core assembly. The air gap, formed by spacers 12,in the magnetic path eliminates the necessity of a reset winding and thelike for demagnetization of the core between pulses.

In accordance with the illustrated embodiment of the invention, theinsulating-winding bobbin 8 is generally a doughnut shaped or annularmember having an inner diameter corresponding substantially to thecorresponding circular configuration of the cores 10 and 11. The bobbin8 encircles the side portions and spans the air gap forming spacer 12and is formed with a generally U-shaped cross section defining anoutwardly opening win-ding groove 14 on its outer periphery. Groove 14extends radially inwardly toward the core portions and completelyencloses the secondary winding 7 on three sides. The bobbin 8 is formedof a suitable insulating thermoplastic or other insulating materialwhich is particularly adapted for insulating the high voltages generatedbetween the secondary and ground, as more fully discussed hereinafter.

The secondary winding 7 is a multiple-turn winding formed fromcontinuous lengths of conductive tape 15 and insulating tape 16, asshown most clearly in FIG. 4. The width of the tapes 15 and 16substantially corresponds to the width of the winding groove 14 and thetapes are wound in a spiral manner .to substantially and completelyfillthe groove 14. The groove 14 is filled with a suitable impregnatingmaterial 17 such as an epoxy resin to essentially eliminate all voidswithin the groove and the secondary winding. The width of the secondarymay be slightly less than the groove 14 to permit ready introduction ofthe impregnating material. The secondary winding 7 includes asubstantially greater number of turns than the primary winding 9' forincreasing or stepping up the voltage of the pulse fed to the gap 3 ofspark plug 1. An outer covering or coating 18 is preferably applied tothe outer surface of the bobbin'8. The coating may be a semi-conductoror other resistive material or a non-continuous conductive material toform a ground plane for the distributed capacity associated withsecondary winding 7. This will essen tially eliminate and preventexternal corona.

In practice, winding 7 may be formed, for example, from separate tapes15 and 16, from a conductive foil base as the conductive strip with aninsulating layer coated on one or both sides, or from an insulating basecoated on one or both sides with a conducting layer. The conductive tapemay for example be formed of an insulating Mylar of .00025 inch thickand 7 inch wide 4 and an aluminum foil of .00017 inch thick and inchwide thereon. The lesser width of the conducting layer is preferably toprotect against a short circuited turn. The secondary winding mayincludes 1000 turns with three turns or the like in the primary winding.

An integral insulating tower 19 projects axially from the bobbin 8adjacent the inner convolutions or turns of the secondary with aninsulated high voltage lead 20 extending therefrom. The conductor withinlead 20 is connected to a wire or strip 21 connected to an inner turn orconvolution of the secondary winding 7 within the groove 14 and wedgedor otherwise held within the opening of tower 19 for connection withlead 20. Thus, in practice a couple of inner turns of the secondarywinding 7 may be formed with the wire 21 inserted and the subsequentturns completed, generally in accordance with known practice in formingof condensers. The outer end of the winding 7 is connected by a lead 22to one side of the primary '7 which is connected to ground to completethe connection of the secondary winding 7 in the circuit of spark plug1, as schematically shown in FIG. 1.

Materials and winding techniques employed in connection with thesecondary transformer winding will generally be similar to knowntechniques employed in winding capacitors. Generally, the high voltageand ground connection of the secondary winding 7 can be made in"accordance with known condenser concepts.

The insulation of the secondary winding 7 from ground is provided by thebobbin 8 which provides a continuous homogeneous mass of insulatingmaterial between the high voltage inner end of the secondary winding 7and all of the grounded surfaces. The bobbin 8 is formed of suitablematerial such as thermoplastic or epoxy resin which can withstand thehigh voltages generated in the secondary winding 7 and with which theimpregnating material bonds, shown along the edges as at 17 of secondarywinding 7 for purposes of illustration. The impregnating material, aspreviously noted is to fill all voids within the groove. The insulationthus provides maximum insulating characteristics and for any given sizeprovides maximum protection against breakdown and thus permits highersecondary voltages. The present invention is particularly directed tothe use of a transformer in a pulse system generally similar to highfrequency voltage systems. Dielectric failure of a continuous insulatingmass is minimized and therefore provides a highly satisfactory andreliable operation for rapidly rising pulses.

In the selected applications generally where size and weightconsiderations are paramount, the inner corners of the groove 14- may berounded as at 23 shown substantially enlarged for illustrative purposesand provided with a high resistivity or non-continuous conductivecoating in any desired or well known manner. The rounding of the corners23 and the provision of the coating will prevent corona effects andreduce concentration of dielectric stress and thereby increases thereliability of the total system. A similar result can be'obtained'byemploying a thicker conductive tape with rounded corners for theinnermost turn or turns or winding of the secondary on a tubular formwhich serves as the initial turn and which has the ends rounded.

As most clearly shown in FIG. '3, the axial depth of the bobbin 8 isslightly less than the depth of the winding window formed by theU-shaped cores 10 and 11 and defines a small ring-like winding space tothe one side of the bobbin within which the primary winding 9 is wound.The primary winding 9 is wound from a continuous conductor which isillustrated as having a rectangular cross section in FIG. 3 althoughround or other shapes may be employed. A suitable low voltage covering24 such as a cloth tape encloses the winding 9 to prevent grounding. Apair of integral input terminals or input leads 25 projects axially ofbobbin 8 and is connected to the pulse source 5 for selectiveenergization of the primary winding 9 and transfer of energy to the highvoltage secondary winding 7. The one lead is grounded and connected bylead 22 to the ground side of the secondary Winding 7 in the illustratedembodiment of the invention.

In operation, the illustrated ignition system has an ignition pulsewhich is applied to the primary Winding 9 as a low voltage, rapidlyrising pulse and thereby generates a high voltage pulse in the secondarywinding 7. The high voltage pulse is impressed across the spark gap 3 ofthe spark plug 1 and results in a spark for igniting a fuel mixture.

The present invention is particularly unique in providing a very highstep-up voltage in a small compact unit which can be employed in aninternal-combustion engine for automobiles, outboard motors and the likewhere it is subject to substantial vibration and rough usage. The bobbin8 provides homogeneous insulation as previously noted to minimizebreakdown of the insulation and loss of firing signal pulses. Thevoltage rises very rapidly with the present invention as a result ofrelatively close coupling of the primary to the secondary and lowsecondary distributed capacitance. The secondary windings may be woundof two insulated tapes producing a bifilar winding which is particularlyuseful in systems for exciting magnetrons and the like. The connectionto the high voltage lead 18 at an inner convolution provides maximuminsulation to all grounded surfaces and also minimizes the capacitanceof the high voltage end of the winding 7 to ground. Leakage inductancecan be further reduced, if necessary or desired, by placing a secondprimary winding on the opposite side of the bobbin and connecting it inseries or parallel with the'illustrated primary winding 9. Similarly, apair of secondary windings may be disposed on opposite sides of aprimary winding.

Although shown in connection with a generally rectangular corestructure, the present invention may employ any other suitable corestructure providing close coupling of the primary and secondary. In someapplications, the core structure may be eliminated. For example, inFIGS. 5 and 6, an alternative core construction is shown wherein a pottype core assembly 26 is provided for supporting primary and secondarywindings, as follows.

A pancake type primary winding 27 and a high voltage multiple turnsecondary winding 28 carried within a bobbin 29 are provided generallyas in the structure previously described with respect to FIGS. 1-4. InFIGS. 5 and 6, however, the core assembly includes a pot type core 30which is generally a cup-shaped member open at one end and having acentral pole 3 1 integrally formed and projecting outwardly inaccordance with the thickness of the core such that a vertical crosssection, as

shown in FIG. 5, defines anE-coniiguration. A core' cover 32 closes theopen end of the pot core 30 and is interconnected to provide a unitaryunit construction by a bolt and nut attachment unit 33 which extendsthrough aligned apertures in the cover i3 2 and the pole 31. The crosssection of the annular winding space generally corresponds to a windingWindow of FIGS. l4 such that the bobbin 29 and the primary winding =27substantially fill the winding space. A terminal aperture 34 is providedin the base of the core 30 with a terminal tower 35 projecting outwardlyfrom the bobbin 29 for winding 2-8. The primary terminals 36 of winding-27 project downwardly through suitable apertures in the core cover 32for interconnection with the ignition circuit or the like.

In operation, the embodiment of FIGS. 5 'and 6 functions substantiallyin the same manner as the previous embodiment to provide a compact andreliable transformer for transforming of low voltage, rapidly risingpulses to high voltage, rapidly rising pulses. Thus, the output voltagepulse may have a peak value in the order of 30 to 100 kilovolts or moreand a period of microseconds or less. A 1000 or more turns may be usedin the secondary to reduce the voltage diiference per turn. The

solid, insulating bobbin provides adequate insulation to ground,particularly between the highest voltage innermost convolution andground. i

As previously noted, the bobbin may constitute a winding form withinwhich the secondary is wound, it may be preformed in two or more partsand assembled with a previously wound secondary or it may be molded orotherwise cast about the secondary within the scope of the presentinvention as defined in the accompanying claims.

The present invention thus provides an efiicient, high frequency pulsetransformer which is particularly useful in producing firing pulses ofhigh voltage and short duration.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. A transformer structure comprising,

a magnetic core unit having a winding opening and an adjacent magneticwinding portion,

a secondary winding insulator of a high voltage insulating materialencircling the core unit winding por tion at the opening and having asecondary Winding groove within the insulator and opening generallyoutwardly of the winding portion,

a secondary winding within the groove and having a high voltage terminalat the base of the groove, and

'a primary winding means encircling a portion of the core unitimmediately adjacent at least one side of the Winding insulator.

2. The transformer of claim 1, having said secondary winding in the formof a tape conductor of a substantially greater width than thickness andcoated with an insulating material on at least one face of the tapeconductor, the width of the tape substantially corresponding to thewidth of the groove, and said tape condu'ctor located within the grooveto form a spiral shaped winding essential-1y completely filling thegroove with the high voltage turn adjacent the base of the groove andthe low voltage turn adjacent the entrance ofthe groove.

3. The transformer of claim 2 wherein the winding insulator includes ahigh voltage terminal tower projecting axially of the insulator adjacentthe base of the groove and a high voltage terminal secured to an innerconvolution of the winding.

4. A transformer structure comprising,

a magnetic core unit having a winding Window defined in part by amagnetic winding portion,

a preformed secondary winding insulator of insulating materialencircling said winding portion and having a Winding groove in anexterior surface,

a conductive tape means wound upon itself with insulation between theadjacent convoluti-ons to form a spiral secondary winding and woundwithin the groove with an axis parallel to the Winding port-ion andhaving a high voltage terminal at the base of the groove, and

a generally fl'at, pancake-type primary winding encircling the Windingportion immediately adjacent one side of the winding insulator, saidwinding having a relatively few turns compared to the secondary windingand extending generally coextensively of the secondary Winding.

5. The structure of claim 4 wherein the magnetic core unit includes apair of U-shaped cores secured together to form a rectangular core unit.

6. The structure of claim 5 having a non-magnetic gap between at leastone pair of adjacent ends of the cores and having the insulatorencircling the air gap and both portions of the cores adjacent theencircled air gap.

7. The structure of claim 5 wherein the insulator and cores havecorresponding mating dimensions.

l 8. The structure of claim 4 wherein the magnetic core unit includes apot core open at one end and having a base with a central pole and anouter annular wall extending from the base and defining an annularopening substantially corresponding to the diameters of the insulatorand primary Winding, and

a cover secured to the open end of the pot core with the primary windingdisposed between the cover and the insulator.

9. The transformer of claim 4 for transmitting a high voltage pulsehaving,

said secondary Winding tape being of a substantially greater Width thanthickness and coated with an insulating material on at least onesurface, the width of the tape being slightly smaller than the width ofthe groove, and an impregnating material within the groove toessentially completely eliminate all voids within the groove. e 10. Ahigh voltage pulse transformer for transmitting a pulse with a rapidrise time, comprising a magnetic core defining a substantiallycontinuous magnetic path about a winding opening,

an annular winding bobbin having a central opening corresponding to awinding portion of the core at the winding opening and being assembledin mating relation therewith, the bobbin being extended outwardly fromthe winding portion for the width of the window and having a generallyrectangular winding groove in the outer periphery thereof,

a primary winding encircling the winding portion of the core,

a secondary Winding located within and substantially tfilling the groovein the form of a continuous spirally wound conductive toil havinginsulation between eachturn to provide a multiple tur-n spiral windingof a single conductor width, said winding being wound I about the bobbinwith the inner turns at the base of a the groove and the outer turns atthe entrance to the groove, and

a high voltage terminal secured to an inner turn of the secondarywinding.

11. A high voltage pulse transformer for transmitting a pulse with arapid rise time, comprising i a magnetic core unit defining "asubstantially continuou magnetic path about a winding opening and having4 a Winding portion contiguous said opening,

a pancake primary winding including a spirally wound conductorencircling the core adjacent one end of the winding opening,

a winding bobbin having a central opening corresponding to the windingportion of the core and arranged in mating relation therewith, saidbobbin extending outwardly from the winding portion across the openingto the opposite portion of the 'core and having a generally rectangularWinding groove in the outer portion, the inner corner's of the groovebeing curved and covered with an integral high resistivity coating toreduce local concentration of dielectric stress, the depth of the bobbinbeing generally less than the depth of the Winding opening by thethickness of the pancake primary coil,

a secondary winding located within and filling the groove in the form ofa continuous conductive toil having continuous insulation on at leastone "face, said fOll having -a width substantially corresponding to thegroove to dill the groove with a multiple turn winding of a singleconductor width baving inner turns at the base of the groove, and

a high voltage terminal secured to an inner secondary turn.

12. The structure of claim 11 wherein,

the magnetic core unit includes a pair of U-shaped cores securedtogether to form a generally rectangular core unit with a non-magneticgap between at least one pair of adjacent ends of the 'U-shaped coresand having the bobbin encircling the gap and both portions of the coresadjacent the gap.

13. The structure of claim '11 wherein the bobbin is an annular memberand the magnetic core unit includes a pot core having 'a central poleand an outer annular Wall defining an annular chamber having one andopen, said chamber substantially corresponding to the bobbin and of asomewhat greater depth, and

a cover secured to the pot core overlying said open end of the chamberand having the primary winding disposed between the cover and thebobbin.

14. The structure of claim 13 wherein,

the outer surface of the bobbin is covered with an integral highresistivity coating.

References Cited by the Examiner UNITED STATES PATENTS 2,064,771 12/1936VOgt 336178 X 2,762,019 9/1956 Short et al 336l78 X 2,867,785 1/1959Crumbliss 3'36-178 2,998,583 8/ 1961 Worcester '336-223 X LEWIS H.MYERS, Primary Examiner.

ROBERT K. scHAEFE-R, Examiner.

C. TORR'ES, Assistant Examiner.

1. A TRANSFORMER STRUCTURE COMPRISING, A MAGNETIC CORE UNIT HAVING AWINDING OPENING AND AN ADJACENT MAGNETIC WINDING PORTION, A SECONDARYWINDING INSULATOR OF A HIGH VOLTAGE INSULATING MATERIAL ENCIRCLING THECORE UNIT WINDING PORTION AT THE OPENING AND HAVING A SECONDARY WINDINGGROOVE WITHIN THE INSULATOR AND OPENING GENERALLY OUTWARDLY OF THEWINDING PORTION, A SECONDARY WINDING WITHIN THE GROOVE AND HAVING A HIGHVOLTAGE TERMINAL AT THE BASE OF THE GROOVE, AND A PRIMARY WINDING MEANSENCIRCLING A PORTION OF THE CORE UNIT IMMEDIATELY ADJACENT AT LEAST ONESIDE OF THE WINDING INSULATOR.